Don't require refout to produce marker (#2180)
Don't require refout to produce marker (#2180)

Required for dotnet/project-system#2254.

Given:

ClassLibrary1 -> ClassLibrary2 -> ConsoleApp1

Where ClassLibrary2 references ClassLibrary1 and ConsoleApp1 references only ClassLibrary2. And both ClassLibrary1 and ClassLibrary2 output ref assemblies.

So we do a full build, touch the implementation of ClassLibrary1, and then build.

- ClassLibrary1 is out of date, so it builds ClassLibrary1.dll. However, since the interface didn't change, ref\ClassLibrary1.dll is not touched.
- ClassLibrary2 is out of date because input ClassLibrary1.dll is now newer than output ClassLibrary2.dll. So it builds. However, since all inputs of CoreCompile are up to date (since ref\ClassLibrary1.dll isn't newer than ClassLibrary2.dll), we don't rebuild ClassLibrary2.dll or ref\ClassLibrary2.dll. We just copy the updated implementation of ClassLibrary1, and touch ClassLibrary2.marker.
- ConsoleApp1 is out of date because input ClassLibrary2.marker is now newer than output ConsoleApp1.dll. So it builds. However, since all inputs of CoreCompile are up to date (since ref\ClassLibrary2.dll isn't newer than ConsoleApp1.dll), we don't rebuild ConsoleApp1.dll. We just copy the updated implementation of ClassLibrary1. There's no marker for ConsoleApp1.

The problem in this situation is that now if you do another build, ConsoleApp1 will fail the up to date check and copy ref\ClassLibrary2.dll again, because ClassLibrary2.marker will always be newer than ConsoleApp1.dll. If ConsoleApp1 had a marker, though, then we would just compare the markers and know it was up to date.
jeffkl/msbuild,Microsoft/msbuild,chipitsine/msbuild,AndyGerlicher/msbuild,AndyGerlicher/msbuild,cdmihai/msbuild,mono/msbuild,jeffkl/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,cdmihai/msbuild,chipitsine/msbuild,AndyGerlicher/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,mono/msbuild,chipitsine/msbuild,jeffkl/msbuild,Microsoft/msbuild,mono/msbuild,Microsoft/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,chipitsine/msbuild,sean-gilliam/msbuild,cdmihai/msbuild,Microsoft/msbuild,mono/msbuild,jeffkl/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild